1. Field of the Invention
The invention relates to a power tool having a three-phase brushless motor, and more particularly, to a technique for controlling an output characteristic of the motor for the power tool.
2. Description of the Related Art
A stator of a brushless motor is known to have a plurality of slots provided on its inner circumferential surface side on which a rotor including a permanent magnet is disposed. A stator winding is formed by winding coils through the slots and connecting them in series or in parallel. When driving current of the motor is supplied to the coils of the stator winding, a magnetic field is generated around the coils and the magnetic field interacts with the field generated by the permanent magnet, so that a rotor is driven.
The known stator winding includes the same number of turns of coils wound through the slots, which number is selected depending on the desired output characteristic of the motor or the desired torque or rotation speed. If a different number of turns is selected for each of the coils forming the stator winding, the magnetic field which is generated around each coil when the driving current passes through the coil will vary in strength. As a result, variations in the torque of the motor may be caused, so that a vibration problem may arise.
On the other hand, however, the torque output characteristic of the motor may not reach to a desired level if all of the coils are wound “n” turns, while it may exceed a desired level if all of the coils are wound “n+1” turns.
In this connection, Japanese Utility Model Publication No. 61-3241A discloses a technique to control the number of turns of coils for controlling the motor characteristic. According to the known art, a subsidiary stator winding is provided in the same phase as a main stator winding and the number of slots of the subsidiary stator winding or the number of turns per slot can be changed. However, in this known art, a subsidiary stator winding is used in addition to the main stator winding, which disadvantageously limits a space for winding the main stator winding.